This invention relates generally to case structures for turbomachinery and more particularly to cases which include bleed flow provisions.
A gas turbine engine includes a compressor that provides pressurized air to a combustor wherein the air is mixed with fuel and ignited for generating hot combustion gases. These gases flow downstream to one or more turbines that extract energy therefrom to power the compressor and provide useful work such as powering an aircraft in flight. The compressor typically includes multiple stages, each of which comprises a row of stationary stator vanes and an adjacent row of compressor blades attached to a rotor. A portion of compressed air from one or more stages may be extracted for turbine section cooling, airframe pressurization, anti-icing, or other uses.
A case assembly comprising inner and outer cases surrounds the compressor rotor. To extract large volumes of bleed air, an annular bleed slot is defined between the outer case and the inner case. The inner case is less massive than the rotor and is bathed in compressed, heated air. Accordingly, the inner case has a rapid thermal response, that is, it expands or contracts more rapidly than the rotor as the temperature of the air flow changes. This results in phenomena known as thrust droops and rubs, which cause loss of efficiency and possible engine damage. Prior art solutions have attempted to tie the inner and outer cases together to provide better support of the inner case. However, the prior art designs include cantilevered shroud sections that allow undesired radial movement in the portions of the inner case surrounding one or more blade stages.
Accordingly, there is a need for a compressor bleed case having a reduced thermal response.
The above-mentioned need is met by the present invention, which provides a bleed case assembly comprising an annular outer case and an inner case. The inner case comprises an annular shroud portion having a radially outer surface opposite a radially inner surface, and a generally planar flange extending radially outwardly from the radially outer surface. The flange includes a circumferential array of bleed openings formed therethrough. The flange is attached to the outer case so as to reduce the thermal response of the inner case.
The present invention and its advantages over the prior art will become apparent upon reading the following detailed description and the appended claims with reference to the accompanying drawings.